U.S. patent number 7,758,842 [Application Number 10/570,074] was granted by the patent office on 2010-07-20 for filmy graphite and process for producing the same.
This patent grant is currently assigned to Kaneka Corporation. Invention is credited to Kiyokazu Akahori, Mutsuaki Murakami, Yasushi Nishikawa.
United States Patent |
7,758,842 |
Nishikawa , et al. |
July 20, 2010 |
Filmy graphite and process for producing the same
Abstract
A process for producing a filmy graphite includes the steps of
forming a polyimide film having a birefringence of 0.12 or more and
heat-treating the polyimide film at 2,400.degree. C. or higher.
Inventors: |
Nishikawa; Yasushi (Osaka,
JP), Murakami; Mutsuaki (Osaka, JP),
Akahori; Kiyokazu (Shiga, JP) |
Assignee: |
Kaneka Corporation (Osaka-Shi,
JP)
|
Family
ID: |
34260120 |
Appl.
No.: |
10/570,074 |
Filed: |
September 2, 2003 |
PCT
Filed: |
September 02, 2003 |
PCT No.: |
PCT/JP03/11221 |
371(c)(1),(2),(4) Date: |
March 01, 2006 |
PCT
Pub. No.: |
WO2005/023713 |
PCT
Pub. Date: |
March 17, 2005 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20070032589 A1 |
Feb 8, 2007 |
|
Current U.S.
Class: |
423/448; 528/170;
524/495 |
Current CPC
Class: |
C04B
35/522 (20130101); C01B 32/205 (20170801); C04B
35/524 (20130101); C08G 73/1053 (20130101); C01B
32/20 (20170801); C04B 2235/9607 (20130101); Y10T
428/31721 (20150401); C04B 2235/96 (20130101); C04B
2235/656 (20130101) |
Current International
Class: |
C01B
31/04 (20060101); B60C 1/00 (20060101) |
Field of
Search: |
;528/170,353
;423/448 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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63-197628 |
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Aug 1988 |
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JP |
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05-132360 |
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May 1993 |
|
JP |
|
05-237928 |
|
Sep 1993 |
|
JP |
|
07-109171 |
|
Apr 1995 |
|
JP |
|
07-109171 |
|
Apr 1995 |
|
JP |
|
2000-044220 |
|
Feb 2000 |
|
JP |
|
2000-063543 |
|
Feb 2000 |
|
JP |
|
2001-072781 |
|
Mar 2001 |
|
JP |
|
2003-073473 |
|
Mar 2003 |
|
JP |
|
2003-165850 |
|
Jun 2003 |
|
JP |
|
Other References
Murakami, M. et al., "High-Quality and Highly Oriented Graphite
Block from Polycondensation Polymer Films," Carbon, vol. 30, No. 2,
1992, pp. 255-262. cited by other .
Murakami, Matsuaki et al., "New Highly Oriented Graphite Crystals
for Radiation Optics," Hoshako, vol. 6, No. 3, 1993, pp. 43-50.
cited by other .
Toshiharu Hoshi et al., "Super Graphite," National Technical
Report, vol. 40, No. 1, Feb. 1994, pp. 74-80. cited by other .
Plastic Jiten, Kabusiki Kaisha Asakura Shoten, Mar. 1, 1992, p.
620, Table 3.8.5. cited by other .
Catalogue of Kapton. cited by other .
ASTM D696-91, Standard Test Method for Coefficient of Linear
Thermal Expansion of Plastics Between -30.degree. C and 30.degree.
C. cited by other.
|
Primary Examiner: Hendrickson; Stuart
Assistant Examiner: Rump; Richard M
Attorney, Agent or Firm: Marshall, Gerstein & Borun
LLP
Claims
The invention claimed is:
1. A process for producing a filmy graphite comprising the steps
of: forming a polyimide film having a thickness of 75 .mu.m or less
and a birefringence of 0.12 or more; and heat-treating the
polyimide film at 2,700.degree. C. or higher, wherein the filmy
graphite has an electrical conductivity of 9,200 S/cm or more.
2. A process for producing a filmy graphite comprising the steps
of: forming a polyimide film having a thickness of 75 .mu.m or less
and a mean coefficient of linear expansion of less than
2.5.times.10.sup.-5/.degree. C. in a range of 100.degree. C. to
200.degree. C., the mean coefficient of linear expansion being in a
planar direction of the film; and heat-treating the polyimide film
at 2,400.degree. C. or higher, wherein the filmy graphite has an
electrical conductivity of 12,000 S/cm or more.
3. A process for producing a filmy graphite comprising the steps
of: forming a polyimide film having a thickness of 75 .mu.m or less
and a mean coefficient of linear expansion of less than
2.5.times.10.sup.-5/.degree. C. in a range of 100.degree. C. to
200.degree. C., the mean coefficient of linear expansion being in a
planar direction of the film, and having a birefringence of 0.12 or
more; and heat-treating the polyimide film at 2,400.degree. C. or
higher, wherein the filmy graphite has an electrical conductivity
of 12,000 S/cm or more.
4. A process for producing a filmy graphite comprising the steps
of: forming a polyimide film having a thickness of 100 .mu.m or
less and a birefringence of 0.12 or more; and heat-treating the
polyimide film at 2,700.degree. C. or higher, wherein the filmy
graphite has an electrical conductivity of 8,000 S/cm or more.
5. The process for producing a filmy graphite according to claim 4,
wherein the filmy graphite has an electrical conductivity of 9,300
S/cm or more.
6. The process for producing a filmy graphite according to claim 4,
wherein the filmy graphite has an electrical conductivity of 9,700
S/cm or more.
7. A process for producing a filmy graphite comprising the steps
of: forming a polyimide film having a thickness of 50 .mu.m or less
and a mean coefficient of linear expansion of less than
2.5.times.10.sup.-5/.degree. C. in a range of 100.degree. C. to
200.degree. C. birefringence of 0.12 or more; and heat-treating the
polyimide film at 2,400.degree. C. or higher, wherein the filmy
graphite has an electrical conductivity of 16,000 S/cm or more.
8. A process for producing a filmy graphite comprising the steps
of: forming a polyimide film having a thickness of 200 .mu.m or
less and a birefringence of 0.12 or more; and heat-treating the
polyimide film at 2,700.degree. C. or higher, wherein the filmy
graphite has an electrical conductivity of 9,800 S/cm or more.
Description
RELATED APPLICATIONS
This application is a nationalization of PCT application
PCT/JP2003/011221 filed on Sep. 2, 2003, the contents of which are
incorporated herein by reference in their entirety.
TECHNICAL FIELD
The present invention relates to a filmy graphite used as a
heat-dissipating film, a heat-resistant seal, a gasket, a heating
element, or the like, and a process for producing the same.
BACKGROUND ART
Filmy graphites are important as industrial materials because of
their excellent heat resistance, chemical resistance, high thermal
conductivity, and high electrical conductivity, and are widely used
as heat-dissipating materials, heat-resistant sealing materials,
gaskets, heating elements, etc.
As a representative example of a process for producing an
artificial filmy graphite, a process referred to as an "expanded
graphite production process" is known. In this process, natural
graphite is dipped in a mixed solution of concentrated sulfuric
acid and concentrated nitric acid, followed by rapid heating to
produce an artificial graphite. The resulting artificial graphite
is washed to remove the acids and then formed into a film with a
high-pressure press. However, in the filmy graphite thus produced,
strength is low and other physical properties are insufficient.
Moreover, the residual acids also give rise to a problem.
In order to overcome these problems, a process has been developed
in which a special polymer film is graphitized by direct heat
treatment (hereinafter, referred to as a "polymer graphitization
process"). Examples of the polymer film used for this purpose
include films containing polyoxadiazole, polyimide,
polyphenylenevinylene, polybenzimidazole, polybenzoxazole,
polythiazole, or polyamide. The polymer graphitization process is a
process which is far simpler than the conventional expanded
graphite production process, in which mixture of impurities, such
as acids, does not essentially occur, and which is capable of
achieving excellent thermal conductivity and electrical
conductivity close to those of single crystal graphite (refer to
Japanese Unexamined Patent Application Publication Nos. 60-181129,
7-109171, and 61-275116).
However, the polymer graphitization process has two problems.
First, it is difficult to obtain a thick filmy graphite compared
with the expanded graphite production process. Although various
attempts have been made to improve such a problem, as it now
stands, transformation into a quality graphite is possible only
when the thickness of the starting material film is up to about 50
.mu.m.
Secondly, the graphitization requires long-time heat treatment at
extremely high temperatures. In general, transformation into a
quality graphite requires heat treatment in a temperature range of
2,800.degree. C. or higher for at least 30 minutes.
DISCLOSURE OF INVENTION
In view of the problems associated with the conventional polymer
graphitization process, it is an object of the present invention to
provide a thick filmy graphite having excellent physical
properties, the filmy graphite being produced by short-time heat
treatment at relatively low temperatures.
In order to overcome the problems described above, the present
inventors have taken notice of a polyimide which represents a
graphitizable polymer, and it has been attempted to graphitize
various polyimide films. As a result, it has been found that by
controlling the molecular structure and molecular orientation of
the polyimide, transformation into a quality graphite is enabled.
More specifically, it has been found that the birefringence or
coefficient of linear expansion, which is a physical property of a
polyimide film, can be the most direct indicator of whether the
polyimide film can be transformed into a quality graphite. Here,
the coefficient of linear expansion is defined as a coefficient of
linear expansion in a direction parallel to the film plane.
That is, according to the present invention, a process for
producing a filmy graphite includes the steps of forming a
polyimide film having a birefringence of 0.12 or more and
heat-treating the polyimide film at 2,400.degree. C. or higher.
Alternatively, a process for producing a filmy graphite may include
the steps of forming a polyimide film having a mean coefficient of
linear expansion of less than 2.5.times.10.sup.-5/.degree. C. in a
range of 100.degree. C. to 200.degree. C., the mean coefficient of
linear expansion being in a planar direction of the film, and
heat-treating the polyimide film at 2,400.degree. C. or higher.
Preferably, a process for producing a filmy graphite includes the
steps of forming a polyimide film having a mean coefficient of
linear expansion of less than 2.5.times.10.sup.-5/.degree. C. in a
range of 100.degree. C. to 200.degree. C., the mean coefficient of
linear expansion being in a planar direction of the film, and
having a birefringence of 0.12 or more, and heat-treating the
polyimide film at 2,400.degree. C. or higher.
In the process for producing the filmy graphite, the polyimide film
may be formed using, as a starting material, an acid dianhydride
represented by chemical formula 1:
##STR00001## wherein R.sub.1 represents any one of divalent organic
groups represented by chemical formulae 2:
##STR00002## wherein R.sub.2, R.sub.3, R.sub.4, and R.sub.5 each
represent any one selected from the group consisting of --CH.sub.3,
--Cl, --Br, --F, and --OCH.sub.3.
In the process for producing the filmy graphite, preferably, the
polyimide film is formed using, as a starting material, an acid
dianhydride represented by chemical formula 3:
##STR00003##
In the process for producing the filmy graphite, it is also
preferable to form the polyimide film using pyromellitic
dianhydride or p-phenylenediamine as a starting material. In the
process for producing the filmy graphite, the polyimide film may be
formed by treating a polyamic acid, which is a precursor, with a
dehydrating agent and an imidization accelerator. Preferably, the
polyimide film is formed by synthesizing a pre-polymer using a
first diamine and an acid dianhydride, the pre-polymer having the
acid dianhydride moiety at both termini, synthesizing a polyamic
acid by allowing the pre-polymer to react with a second diamine,
and imidizing the polyamic acid.
An artificial filmy graphite according to the present invention can
have a thickness of 30 .mu.m or more and a thermal diffusivity of
8.5.times.10.sup.-4 m.sup.2/s or more. Preferably, the artificial
filmy graphite can have a thickness of 3 .mu.m or more and a
thermal diffusivity of 10.times.10.sup.-4 m.sup.2/s or more.
The artificial filmy graphite can have a thickness of 30 .mu.m or
more and an electrical conductivity of 8.5.times.10.sup.4 Scm or
more. In the filmy graphite, the ratio of electrical resistance at
77 K to that at room temperature can be 1.5 or less, and the ratio
of electrical resistance at 4 K to that at room temperature can be
1.4 or less.
The artificial filmy graphite can have a thickness of 30 .mu.m or
more and a density of 2.15 g/mm.sup.3 or more. In the artificial
filmy graphite having a thickness of 30 .mu.m or more, when a light
beam with a diameter of 10 .mu.m is applied to the center of a
cross section in the thickness direction, with respect to
Raman-scattered light, the ratio of peak height at a wave number of
1,310 cm.sup.-1 to that at a wave number of 1,580 cm.sup.-1 can be
0.35 or less.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view illustrating cutting-out of a specimen for
measuring birefringence of a polyimide film.
FIG. 2 is a perspective view of the specimen for measuring
birefringence cut out as illustrated in FIG. 1.
FIG. 3 is a bright-field image of a filmy graphite in the vicinity
of a surface layer in an example of the present invention, the
image being observed with a transmission electron microscope
(TEM).
FIG. 4 is a lattice image of a filmy graphite in the vicinity of a
surface layer in an example of the present invention, the image
being observed with a TEM.
FIG. 5 is a bright-field image of a filmy graphite in the vicinity
of a center in the thickness direction in an example of the present
invention, the image being observed with a TEM.
FIG. 6 is a lattice image of a filmy graphite in the vicinity of a
center in the thickness direction in an example of the present
invention, the image being observed with a TEM.
FIG. 7 is a bright-field image of a filmy graphite in the vicinity
of a surface layer in a comparative example, the image being
observed with a TEM.
FIG. 8 is a lattice image of a filmy graphite in the vicinity of a
surface layer in a comparative example, the image being observed
with a TEM.
FIG. 9 is a bright-field image of a filmy graphite in the vicinity
of a center in the thickness direction in a comparative example,
the image being observed with a TEM.
FIG. 10 is a lattice image of a filmy graphite in the vicinity of a
center in the thickness direction in a comparative example, the
image being observed with a TEM.
BEST MODE FOR CARRYING OUT THE INVENTION
In a polyimide film used in the present invention, the
birefringence .DELTA.n, which is associated with in-plane
orientation of molecules, is 0.12 or more, preferably 0.14 or more,
and most preferably 0.16 or more, in any in-plane direction of the
film. The birefringence of the film lower than 0.12 indicates
poorer in-plane orientation of molecules of the film.
Graphitization of such a film requires heating to a higher
temperature and a longer heat-treating time. Furthermore, the
resulting filmy graphite tends to have inferior electrical
conductivity, thermal conductivity, and mechanical strength.
On the other hand, at a birefringence of 0.12 or more, in
particular, 0.14 or more, the maximum temperature can be lowered
and the heat-treating time can be shortened. Furthermore, since the
resulting filmy graphite has improved crystal orientation, the
electrical conductivity, thermal conductivity, and mechanical
strength thereof are remarkably improved. Although the reason for
this is not clear, it is assumed that rearrangement of molecules is
required for graphitization, and in the polyimide having excellent
molecular orientation, the required rearrangement of molecules is
minimal, thus enabling graphitization at relatively low
temperatures.
Herein, the term "birefringence" means a difference between a
refractive index in any in-plane direction of a film and a
refractive index in the thickness direction. The birefringence
.DELTA.nx in an in-plane direction X is given by the following
expression: Birefringence .DELTA.nx=(refractive index Nx in
in-plane direction X)-(refractive index Nz in thickness
direction)
FIGS. 1 and 2 illustrate a specific method for measuring
birefringence. Referring to a plan view of FIG. 1, a wedge-shaped
sheet 2 is cut out as a measurement specimen from a film 1. The
wedge-shaped sheet 2 has a long trapezoidal shape with an oblique
line, and one base angle thereof is a right angle. The wedge-shaped
sheet 2 is cut out such that the bottom of the trapezoid is
parallel to the X direction. FIG. 2 is a perspective view of the
measurement specimen 2 thus cut out. Sodium light 4 is applied at
right angles to a cutout cross-section corresponding to the bottom
of the trapezoidal specimen 2, and a cutout cross-section
corresponding to the oblique line of the trapezoidal specimen 2 is
observed with a polarization microscope. Thereby, interference
fringes 5 are observed. The birefringence .DELTA.nx in the in-plane
direction X is represented by the expression:
.DELTA.nx=n.times..lamda./d where n is the number of interference
fringes, .lamda. is the wavelength of sodium D ray, i.e., 589 nm,
and d is the width 3 of the specimen corresponding to the height of
the trapezoid of the specimen 2.
Note that the term "in an in-plane direction X of a film" means
that, for example, the X direction is any one of in-plane
directions of 0 degrees, 45 degrees, 90 degrees, and 135 degrees on
the basis of the direction of flow of materials during the
formation of the film.
Furthermore, the polyimide film used in the present invention,
which is a starting material for the filmy graphite, has a mean
coefficient of linear expansion of less than
2.5.times.10.sup.-5/.degree. C. in a range of 100.degree. C. to
200.degree. C. By using such a polyimide film as a starting
material, transformation into a graphite starts from 2,400.degree.
C. and transformation into a graphite of sufficiently good quality
can take place at 2,700.degree. C. Moreover, in comparison with a
case in which a polyimide film having a coefficient of linear
expansion of 2.5.times.10.sup.-5/.degree. C. or more conventionally
known as a starting material for a filmy graphite is used, in the
polyimide film having a coefficient of linear expansion of less
than 2.5.times.10.sup.-5/.degree. C., transformation into a
graphite is enabled at lower temperatures even at the same
thickness. That is, even if a film that is thicker than the
conventional film is used as a starting material, graphitization is
allowed to proceed easily. More preferably, the coefficient of
linear expansion is 2.0.times.10.sup.-5/.degree. C. or less.
If the coefficient of linear expansion of the film is
2.5.times.10.sup.-5/.degree. C. or more, the change during heat
treatment increases, graphitization becomes disordered, and
brittleness occurs. The resulting filmy graphite tends to have low
electrical conductivity, thermal conductivity, and mechanical
strength. On the other hand, if the coefficient of linear expansion
is less than 2.5.times.10.sup.-5/.degree. C., elongation during
heat treatment is small, graphitization proceeds smoothly, and
brittleness does not occur. As a result, it is possible to obtain a
filmy graphite that is excellent in various properties.
Note that the coefficient of linear expansion of the film is
obtained by the following method. Using a thermomechanical analyzer
(TMA), a specimen is heated to 350.degree. C. at a heating rate of
10.degree. C./min and then air-cooled to room temperature. The
specimen is heated again to 350.degree. C. at a heating rate of
10.degree. C./min, and the mean coefficient of linear expansion at
100.degree. C. to 200.degree. C. during the second heating is
measured. Specifically, using a thermomechanical analyzer (TMA:
SSC/5200H; TMA120C manufactured by Seiko Electronics Industry Co.,
Ltd.), a film specimen with dimensions of 3 mm in width and 20 mm
in length is fixed on a predetermined jig, and measurement is
performed in the tensile mode under a load of 3 g in a nitrogen
atmosphere.
Furthermore, the polyimide film used in the present invention
preferably has an elastic modulus of 350 kgf/mm.sup.2 or more from
the standpoint that graphitization can be more easily performed.
That is, if the elastic modulus is 350 kgf/mm.sup.2 or more, heat
treatment can be performed while applying a tension to the
polyimide film, and it is possible to avoid breakage of the film
resulting from shrinkage of the film during heat treatment. Thus,
it is possible to obtain a filmy graphite that is excellent in
various properties.
Note that the elastic modulus of the film can be measured in
accordance with ASTM-D-882. The polyimide film more preferably has
an elastic modulus of 400 kgf/mm.sup.2 or more, and still more
preferably 500 kgf/mm.sup.2 or more. If the elastic modulus of the
film is less than 350 kgf/mm.sup.2, breakage and deformation easily
occur due to shrinkage of the film during heat treatment, and the
resulting filmy graphite tends to have low electrical conductivity,
thermal conductivity, and mechanical strength.
The polyimide film used in the present invention can be formed by
flow-casting an organic solution of a polyamic acid which is a
precursor of the polyimide onto a support, such as an endless belt
or stainless steel drum, followed by drying and imidization.
A known process can be used as the process for producing the
polyamic acid used in the present invention. Usually, at least one
aromatic acid dianhydride and at least one diamine are dissolved in
substantially equimolar amounts in an organic solvent. The
resulting organic solution is stirred under controlled temperature
conditions until polymerization between the acid dianhydride and
the diamine is completed. Thereby, a polyamic acid is produced.
Such a polyamic acid solution is obtained usually at a
concentration of 5% to 35% by weight, and preferably 10% to 30% by
weight. When the concentration is in such a range, a proper
molecular weight and solution viscosity can be obtained.
As the polymerization method, any of the known methods can be used.
For example, the following polymerization methods (1) to (5) are
preferable.
(1) A method in which an aromatic diamine is dissolved in a polar
organic solvent, and a substantially equimolar amount of an
aromatic tetracarboxylic dianhydride is allowed to react therewith
to perform polymerization.
(2) A method in which an aromatic tetracarboxylic dianhydride and a
less than equimolar amount of an aromatic diamine compound with
respect thereto are allowed to react with each other in a polar
organic solvent to obtain a pre-polymer having acid anhydride
groups at both termini. Subsequently, polymerization is performed
using an aromatic diamine compound so as to be substantially
equimolar with respect to the aromatic tetracarboxylic
dianhydride.
(3) A method in which an aromatic tetracarboxylic dianhydride and
an excess molar amount of an aromatic diamine compound with respect
thereto are allowed to react with each other in a polar organic
solvent to obtain a pre-polymer having amino groups at both
termini. Subsequently, after adding an additional aromatic diamine
compound to the pre-polymer, polymerization is performed using an
aromatic tetracarboxylic dianhydride such that the aromatic
tetracarboxylic dianhydride and the aromatic diamine compound are
substantially equimolar to each other.
(4) A method in which an aromatic tetracarboxylic dianhydride is
dissolved and/or dispersed in a polar organic solvent, and then
polymerization is performed using an aromatic diamine compound so
as to be substantially equimolar to the acid dianhydride.
(5) A method in which a mixture of substantially equimolar amounts
of an aromatic tetracarboxylic dianhydride and an aromatic diamine
are allowed to react with each other in a polar organic solvent to
perform polymerization.
Among these, as in methods (2) and (3), a method in which
sequential control is used by way of a pre-polymer to perform
polymerization is preferable. The reason for this is that by using
sequential control, it is possible to easily obtain a polyimide
film having a low birefringence and a low coefficient of linear
expansion. By heat-treating this polyimide film, it becomes
possible to easily obtain a filmy graphite having excellent
electrical conductivity, thermal conductivity, and mechanical
strength. Furthermore, it is assumed that since the polymerization
reaction is regularly controlled, the overlap between aromatic
rings increases, and graphitization is allowed to proceed easily
even by low-temperature heat treatment.
In the present invention, examples of the acid dianhydride which
can be used for the synthesis of the polyimide include pyromellitic
dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride,
3,3',4,4'-biphenyltetracarboxylic dianhydride,
1,2,5,6-naphthalenetetracarboxylic dianhydride,
2,2',3,3'-biphenyltetracarboxylic dianhydride,
3,3',4,4'-benzophenonetetracarboxylic dianhydride,
2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,
3,4,9,10-perylenetetracarboxylic dianhydride,
bis(3,4-dicarboxyphenyl)propane dianhydride,
1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,
1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride,
bis(2,3-dicarboxyphenyl)methane dianhydride,
bis(3,4-dicarboxyphenyl)ethane dianhydride, oxydiphthalic
dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride,
p-phenylenebis(trimellitic acid monoester anhydride),
ethylenebis(trimellitic acid monoester anhydride), bisphenol A
bis(trimellitic acid monoester anhydride), and analogues thereof.
These may be used alone or in appropriate combination of two or
more.
In the present invention, examples of the diamine which can be used
for the synthesis of the polyimide include 4,4'-oxydianiline,
p-phenylenediamine, 4,4'-diaminodiphenylpropane,
4,4'-diaminodiphenylmethane, benzidine, 3,3'-dichlorobenzidine,
4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone,
4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether,
3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether,
1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane,
4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine
oxide, 4,4'-diaminodiphenyl-N-methylamine,
4,4'-diaminodiphenyl-N-phenylamine,
1,4-diaminobenzene(p-phenylenediamine), 1,3-diaminobenzene,
1,2-diaminobenzene, and analogues thereof. These may be used alone
or in appropriate combination of two or more.
In particular, from the standpoint that the coefficient of linear
expansion can be decreased, the elastic modulus can be increased,
and the birefringence can be increased, use of an acid dianhydride
represented by chemical formula 1 below as a starting material is
preferable in the production of the polyimide film in the present
invention.
##STR00004## In the formula, R.sub.1 represents any one of divalent
organic groups represented by chemical formulae 2:
##STR00005## wherein R.sub.2, R.sub.3, R.sub.4, and R.sub.5 each
represent any one selected from the group consisting of --CH.sub.3,
--Cl, --Br, --F, and --CH.sub.3O.
By using the acid dianhydride described above, it is possible to
obtain a polyimide film having a relatively low coefficient of
water absorption, which is also preferable from the standpoint that
foaming due to moisture can be prevented in the graphitization
process.
In particular, use of any one of the benzene nucleus-containing
organic groups represented by chemical formulae 2 as R.sub.1 in the
acid dianhydride is preferable from the standpoint that the
resulting polyimide film has high molecular orientation, a low
coefficient of linear expansion, a high elastic modulus, a high
birefringence, and a low coefficient of water absorption.
An acid dianhydride represented by molecular formula 3 below may be
used as a starting material in the synthesis of the polyimide in
the present invention to further decrease the coefficient of linear
expansion, increase the elastic modulus, increase the
birefringence, and decrease the coefficient of water
absorption.
##STR00006##
In particular, with respect to a polyimide film produced using, as
a starting material, an acid dianhydride having a structure in
which benzene rings are linearly bonded by two or more ester bonds,
although folded chains are involved, a highly linear conformation
is easily formed as a whole, and the polyimide film has a
relatively rigid property. As a result, by using this starting
material, it is possible to decrease the coefficient of linear
expansion of the polyimide film, for example, to
1.5.times.10.sup.-5/.degree. C. or less. In addition, the elastic
modulus can be increased to 500 kgf/mm.sup.2 or more, and the
coefficient of water absorption can be decreased to 1.5% or
less.
The polyimide of the present invention is preferably synthesized
using p-phenylenediamine as a starting material to further decrease
the coefficient of linear expansion, increase the elastic modulus,
and increase the birefringence.
In the present invention, the acid dianhydride most suitably used
for the synthesis of the polyimide film includes pyromellitic
dianhydride and/or p-phenylenebis(trimellitic acid monoester
dianhydride) represented by (Chemical Formula 3). The number of
moles of one of these or both is preferably 40 mole percent or
more, more preferably 50 mole percent or more, even more preferably
70 mole percent or more, and still more preferably 80 mole percent
or more relative to the total acid dianhydride content. If the
amount of use of these acid dianhydrides is less than 40 mole
percent, the resulting polyimide film tends to have an increased
coefficient of linear expansion, a decreased elastic modulus, and a
decreased birefringence.
Furthermore, in the present invention, the diamine most suitably
used for the synthesis of the polyimide includes 4,4'-oxydianiline
and p-phenylenediamine. The number of moles of one of these or both
is preferably 40 mole percent or more, more preferably 50 mole
percent or more, even more preferably 70 mole percent or more, and
still more preferably 80 mole percent or more relative to the total
diamine content. Furthermore, p-phenylenediamine is included
preferably in an amount of 10 mole percent or more, more preferably
20 mole percent or more, even more preferably 30 mole percent or
more, and still more preferably 40 mole percent or more. If the
contents of these diamines are below the lower limits of these mole
percent ranges, the resulting polyimide film tends to have an
increased coefficient of linear expansion, a decreased elastic
modulus, and a decreased birefringence. However, if the total
diamine content is entirely composed of p-phenylenediamine, it is
difficult to obtain a thick polyimide film which does not
substantially foam. Therefore, use of 4,4'-oxydianiline is
preferable.
Preferred examples of the solvent for the synthesis of the polyamic
acid include amide solvents, such as N,N-dimethylformamide,
N,N-dimethylacetamide, and N-methyl-2-pyrrolidone, and
N,N-dimethylformamide and N,N-dimethylacetamide are particularly
preferably used.
The polyimide may be produced using either a thermal cure method or
a chemical cure method. In the thermal cure method, a polyamic
acid, which is a precursor, is imidized by heating. In the chemical
cure method, a polyamic acid is imidized using a dehydrating agent
represented by an acid anhydride, such as acetic anhydride, and a
tertiary amine, such as picoline, quinoline, isoquinoline, or
pyridine, as an imidization accelerator. Above all, a tertiary
amine having a higher boiling point, such as isoquinoline, is more
preferable. The reason for this is that such a tertiary amine is
not evaporated in the initial stage of the production process of
the film and tends to exhibit a catalytic effect until the final
step of drying.
In particular, from the standpoints that the resulting film tends
to have a low coefficient of linear expansion, a high elastic
modulus, and a high birefringence and that rapid graphitization is
enabled at relatively low temperatures and a quality graphite can
be obtained, chemical curing is preferable. Furthermore, combined
use of the dehydrating agent and the imidization accelerator is
preferable because the resulting film can have a decreased
coefficient of linear expansion, an increased elastic modulus, and
an increased birefringence. Moreover, in the chemical cure method,
since imidization reaction proceeds more rapidly, the imidization
reaction can be completed for a short period of time in heat
treatment. Thus, the chemical cure method has high productivity and
is industrially advantageous.
In a specific process for producing a film using chemical curing,
first, stoichiometric amounts or more of a dehydrating agent and an
imidization accelerator composed of a catalyst are added to a
polyamic acid solution, the solution is flow-cast or applied onto a
support, e.g., a supporting plate, an organic film, such as PET, a
drum, or an endless belt, so as to be formed into a film, and an
organic solvent is evaporated to obtain a self-supporting film.
Subsequently, the self-supporting film is imidized while drying by
heating to obtain a polyimide film. The heating temperature is
preferably in a range of 150.degree. C. to 550.degree. C.
Although the heating rate is not particularly limited, preferably,
gradual heating is performed continuously or stepwise so that the
highest temperature reaches the predetermined temperature range.
The heating time depends on the thickness of the film and the
highest temperature. In general, the heating time is preferably 10
seconds to 10 minutes after the highest temperature is achieved.
Moreover, it is preferable to include a step of fixing and drawing
the film in order to prevent shrinkage in the production process of
the polyimide film because the resulting film tends to have a small
coefficient of linear expansion, a high elastic modulus, and a high
birefringence.
In the graphitization process of the polyimide film, in the present
invention, the polyimide film, which is a starting material, is
subjected to preheat treatment under reduced pressure or in
nitrogen gas to perform carbonization. The preheating is usually
carried out at about 1,000.degree. C., and for example, when the
temperature is raised at a rate of 10.degree. C./min, preferably,
the film is retained for about 30 minutes in a temperature range of
about 1,000.degree. C. In the stage of temperature rise, in order
to prevent loss of molecular orientation of the starting polymer
film, preferably, a pressure is applied in a direction
perpendicular to the surface of the film to an extent that does not
cause breakage of the film.
Subsequently, the carbonized film is set in a very high temperature
oven to perform graphitization. The graphitization is performed in
an inert gas. As the inert gas, argon is suitable, and addition of
a small amount of helium to argon is more preferable. The heat
treatment temperature required is at least 2,400.degree. C. at the
minimum, and heat treatment is finally performed preferably at a
temperature of 2,700.degree. C. or higher, and more preferably
2,800.degree. C. or higher.
As the heat treatment temperature is increased, transformation into
a quality graphite is more easily enabled. However, in view of
economics, preferably, transformation into a quality graphite is
enabled at temperatures as low as possible. In order to achieve a
very high temperature of 2,500.degree. C. or higher, usually, a
current is directly applied to a graphite heater and heating is
performed using the resulting Joule heat. Deterioration of the
graphite heater advances at 2,700.degree. C. or higher. At
2,800.degree. C., the deterioration rate increases about tenfold,
and at 2,900.degree. C., the deterioration rate increases further
about tenfold. Consequently, it brings about a large economical
advantage to decrease the temperature at which transformation into
a quality graphite is enabled, for example, from 2,800.degree. C.
to 2,700.degree. C., by improving the polymer film as the starting
material. Note that in a generally available industrial oven, the
maximum temperature at which heat treatment can be performed is
limited to 3,000.degree. C.
In the graphitization treatment, the carbonized film produced by
the preheat treatment is transformed so as to have a graphite
structure. During this treatment, cleavage and recombination of
carbon-carbon bonds must occur. In order to cause graphitization at
temperatures as low as possible, it is necessary to allow the
cleavage and recombination to occur at minimum energy. The
molecular orientation of the starting polyimide film affects the
arrangement of carbon atoms in the carbonized film, and the
molecular orientation can produce an effect of decreasing the
energy of cleavage and recombination of carbon-carbon bonds during
graphitization. Consequently, by designing molecules so that high
molecular orientation easily occurs, graphitization at relatively
low temperatures is enabled. By using two-dimensional molecular
orientation parallel to the surface of the film, the effect of the
molecular orientation becomes more remarkable.
The second characteristic of the graphitization reaction is that
graphitization does not easily proceed at low temperatures if the
carbonized film is thick. Consequently, when a thick carbonized
film is graphitized, a state may occur in which the graphite
structure is formed in a surface layer while the graphite structure
is not formed yet in an interior region. The molecular orientation
of the carbonized film promotes graphitization in the interior
region of the film, and as a result, transformation into a quality
graphite is enabled at lower temperatures.
Substantially simultaneous progress of graphitization in the
surface layer and in the interior region of the carbonized film is
also useful in avoiding the situation in which the graphite
structure formed in the surface layer is destroyed by a gas
generated from inside, and graphitization of a thicker film is
enabled. The polyimide film formed in the present invention is
believed to have molecular orientation that is most suitable for
producing such an effect.
As described above, by using the polyimide film formed in the
present invention, it becomes possible to graphitize a film that is
thicker than conventional graphitizable polyimide films.
Specifically, even in a film with a thickness of 200 .mu.m,
transformation into a quality filmy graphite is enabled by
selecting an appropriate heat treatment.
Various examples of the present invention together with several
comparative examples will be described below.
EXAMPLE 1
Pyromellitic dianhydride (4 equivalents) was dissolved in a
solution prepared by dissolving 3 equivalents of 4,4'-oxydianiline
and 1 equivalent of p-phenylenediamine in dimethylformamide (DMF)
to produce a solution containing 18.5% by weight of polyamic
acid.
While cooling the resulting solution, an imidization catalyst
containing 1 equivalent of acetic anhydride and 1 equivalent of
isoquinoline, relative to the carboxylic acid group contained in
the polyamic acid, and DMF was added thereto, followed by
defoaming. Subsequently, the resulting mixed solution was applied
onto an aluminum foil such that a predetermined thickness was
achieved after drying. The mixed solution layer on the aluminum
foil was dried using a hot-air oven and a far-infrared heater.
The drying conditions for achieving a final thickness of 75 .mu.m
were as follows. The mixed solution layer on the aluminum foil was
dried in a hot-air oven at 120.degree. C. for 240 seconds to
produce a self-supporting gel film. The resulting gel film was
stripped off from the aluminum foil and fixed on a frame. The gel
film was dried by heating stepwise in a hot-air oven at 120.degree.
C. for 30 seconds, at 275.degree. C. for 40 seconds, at 400.degree.
C. for 43 seconds, and at 450.degree. C. for 50 seconds, and with a
far-infrared heater at 460.degree. C. for 23 seconds. With respect
to other thicknesses, the firing time was adjusted in proportion to
the thickness. For example, in the case of a film with a thickness
of 25 .mu.m, the firing time was decreased to one third, compared
with the case of 75 .mu.m.
Five types of polyimide films with thicknesses of 25 .mu.m, 50
.mu.n, 75 .mu.m, 100 .mu.m, and 200 .mu.m (Sample A: elastic
modulus 400 kgf/mm.sup.2, coefficient of water absorption>2.0%)
were produced.
Sample A was sandwiched between graphite plates, and using a very
high temperature oven provided with a graphite heater, preliminary
treatment was performed in which the temperature was raised to
1,000.degree. C. at a rate of 16.7.degree. C./min under reduced
pressure. Subsequently, using a very high temperature oven, under a
pressurized argon atmosphere of 0.8 kgf/cm.sup.2, the temperature
was raised to 2,700.degree. C. at a rate of 7.degree. C./min.
Furthermore, under a pressurized argon atmosphere of 0.8
kgf/cm.sup.2, the temperature was raised to 2,800.degree. C., the
maximum temperature, at a rate of 2.degree. C./min, and Sample A
was retained for one hour at the maximum temperature. Cooling was
then performed to obtain filmy graphites.
The progress of graphitization was determined by measuring
electrical conductivity and thermal diffusivity in a planar
direction of the film. That is, higher electrical conductivity and
higher thermal diffusivity indicate increased graphitization. The
results thereof are shown in Table 1. In the case of the polyimide
(Sample A) in Example 1, the heat treatment at 2,700.degree. C.
already causes transformation into quality graphites, and excellent
electrical conductivity and thermal conductivity are exhibited. As
is evident from the results, by using the polyimide of Example 1,
it is possible to graphitize a polyimide film that is thicker than
a polyimide film of conventional Kapton (registered trademark) type
shown in Comparative Example 1 which will be described below, and
transformation into a quality graphite is enabled even at a
temperature of 2,700.degree. C., which is 100.degree. C. lower than
the common graphitization temperature of the Kapton type polyimide
film, i.e., 2,800.degree. C.
The electrical conductivity was measured by the four-terminal
method. Specifically, a filmy graphite sample with a size of about
3 mm.times.6 mm was prepared. After confirming that no breaks or
wrinkles were present with an optical microscope, a pair of outer
electrodes were attached to both ends of the sample using silver
paste, and a pair of inner electrodes were attached inside between
the outer electrodes using silver paste. Using a constant current
source ("Programmable Current Source 220" available from Keithley
Instruments, Inc.), a constant current of 1 mA was applied between
the outer electrodes, and the voltage between the inner electrodes
was measured with a voltmeter ("Nanovoltmeter" available from
Keithley Instruments, Inc.). The electrical conductivity was
calculated according to the expression: (applied current/measured
voltage).times.(distance between inner electrodes/cross-sectional
area of sample).
The thermal diffusivity was measured with a thermal diffusivity
meter using an AC method ("LaserPit" available from ULVAC-RIKO,
Inc.), under an atmosphere of 20.degree. C., at 10 Hz.
TABLE-US-00001 TABLE 1 Starting film Heat treatment Coefficient of
temperature Thickness linear expansion Electrical conductivity
Thermal diffusivity (.degree. C.) (.mu.m)
(.times.10.sup.-5/.degree. C.) Birefringence (S cm) (10.sup.-4
m.sup.2/s) 2700 25 1.8 0.14 11,500 8.4 50 1.8 0.14 11,000 8.3 75
1.9 0.14 10,000 8.1 100 1.9 0.14 9,700 8.0 200 2.0 0.14 9,800 8.0
2800 25 1.8 0.14 12,000 8.7 50 1.8 0.14 11,000 8.5 75 1.9 0.14
11,000 8.5 100 1.9 0.14 10,500 8.5 200 2.0 0.14 10,000 8.5
COMPARATIVE EXAMPLE 1
Pyromellitic dianhydride (1 equivalent) was dissolved in a solution
prepared by dissolving 1 equivalent of 4,4'-oxydianiline in DMF to
produce a solution containing 18.5% by weight of polyamic acid.
While cooling the resulting the resulting solution, an imidization
catalyst containing 1 equivalent of acetic anhydride and 1
equivalent of isoquinoline, relative to the carboxylic acid group
contained in the polyamic acid, and DMF was added thereto, followed
by defoaming. Subsequently, the resulting mixed solution was
applied onto an aluminum foil such that a predetermined thickness
was achieved after drying. The mixed solution layer on the aluminum
foil was dried using a hot-air oven and a far-infrared heater.
The drying conditions for achieving a final thickness of 75 .mu.m
were as follows. The mixed solution layer on the aluminum foil was
dried in a hot-air oven at 120.degree. C. for 240 seconds to
produce a self-supporting gel film. The resulting gel film was
stripped off from the aluminum foil and fixed on a frame. The gel
film was dried by heating stepwise in a hot-air oven at 120.degree.
C. for 30 seconds, at 275.degree. C. for 40 seconds, at 400.degree.
C. for 43 seconds, and at 450.degree. C. for 50 seconds, and with a
far-infrared heater at 460.degree. C. for 23 seconds. With respect
to other thicknesses, the firing time was adjusted in proportion to
the thickness. For example, in the case of a film with a thickness
of 25 .mu.m, the firing time was decreased to one third, compared
with the case of 75 .mu.m.
Five types of conventional polyimide films of typical Kapton
(registered trademark) type with thicknesses of 25 .mu.m, 50 .mu.m,
75 .mu.m, 100 .mu.m, and 200 .mu.m (elastic modulus 300
kgf/mm.sup.2, coefficient of water absorption>2.0%) were
produced. Using these films, graphitization was performed also in
this comparative example by the same method as that in Example
1.
The properties of the filmy graphites produced in Comparative
Example 1 are shown in Table 2. As is evident from Table 2, when
the films with a thickness of 75 .mu.m or more are used, the
resulting graphites have poor electrical conductivity and thermal
diffusivity. Only in the polyimide films with thicknesses of 25
.mu.m and 50 .mu.m, high graphitization is achieved. As is also
evident from Table 2, the properties of the graphitized films
obtained by the heat treatment at 2,700.degree. C. are considerably
inferior to those of the case in which the polyimide (Sample A) of
Example 1 is used.
From the comparison between Comparative Example 1 and Example 1
described above, the superiority of the polyimide of the present
invention in the graphitization reaction is apparent.
TABLE-US-00002 TABLE 2 Starting film Heat treatment Coefficient of
temperature Thickness linear expansion Electrical conductivity
Thermal diffusivity (.degree. C.) (.mu.m)
(.times.10.sup.-5/.degree. C.) Birefringence (S cm) (10.sup.-4
m.sup.2/s) 2700 25 3.2 0.11 9,500 7.2 50 3.1 0.10 9,000 6.3 75 3.2
0.10 5,000 4.0 100 3.1 0.10 1,200 2.0 200 3.1 0.10 800 1.5 2800 25
3.2 0.11 11,500 8.0 50 3.1 0.10 10,000 7.8 75 3.2 0.10 7,000 4.5
100 3.1 0.10 4,500 3.0 200 3.1 0.10 1,000 1.8
EXAMPLE 2
Pyromellitic dianhydride (3 equivalents) was dissolved in a
solution prepared by dissolving 2 equivalents of 4,4'-oxydianiline
and 1 equivalent of p-phenylenediamine in dimethylformamide (DMF)
to produce a solution containing 15% by weight of polyamic
acid.
While cooling the resulting solution, an imidization catalyst
containing 1 equivalent of acetic anhydride and 1 equivalent of
isoquinoline, relative to the carboxylic acid group contained in
the polyamic acid, and DMF was added thereto, followed by
defoaming. Subsequently, the resulting mixed solution was applied
onto an aluminum foil such that a predetermined thickness was
achieved after drying. The mixed solution layer on the aluminum
foil was dried using a hot-air oven and a far-infrared heater.
The drying conditions for achieving a final thickness of 75 .mu.m
were as follows. The mixed solution layer on the aluminum foil was
dried in a hot-air oven at 120.degree. C. for 240 seconds to
produce a self-supporting gel film. The resulting gel film was
stripped off from the aluminum foil and fixed on a frame. The gel
film was dried by heating stepwise in a hot-air oven at 120.degree.
C. for 30 seconds, at 275.degree. C. for 40 seconds, at 400.degree.
C. for 43 seconds, and at 450.degree. C. for 50 seconds, and with a
far-infrared heater at 460.degree. C. for 23 seconds. With respect
to other thicknesses, the firing time was adjusted in proportion to
the thickness. For example, in the case of a film with a thickness
of 25 .mu.m, the firing time was decreased to one third, compared
with the case of 75 .mu.m.
When compared with Example 1, the percentage of p-phenylenediamine,
i.e., a rigid component, was high in Example 2, and the resulting
polyimide films had higher molecular orientation. Consequently, in
the case of thick films, the solvent and the catalyst were caught
in the resin, and foaming easily occurred because of the
evaporation of the solvent of the polyimide film and the
imidization catalyst. In order to prevent foaming, it was necessary
to set the firing time at low temperatures to be sufficiently
long.
Four types of polyimide films with thicknesses of 25 .mu.m, 50
.mu.m, 75 .mu.m, and 100 .mu.m (Sample B: elastic modulus 450
kg/mm.sup.2, coefficient of water absorption>2.0%) were
produced. Using these films, graphitization was performed also in
Example 2 by the same method as that in Example 1.
The properties of the filmy graphites produced in Example 2 are
shown in Table 3. As is evident from comparison between Tables 3
and 1, the properties of the filmy graphites obtained in Example 2
are slightly superior to those in Example 1.
TABLE-US-00003 TABLE 3 Starting film Heat treatment Coefficient of
temperature Thickness linear expansion Electrical conductivity
Thermal diffusivity (.degree. C.) (.mu.m)
(.times.10.sup.-5/.degree. C.) Birefringence (S cm) (10.sup.-4
m.sup.2/s) 2700 25 1.2 0.15 11,500 8.5 50 1.1 0.16 11,000 8.3 75
1.2 0.16 10,000 8.2 100 1.1 0.15 9,700 8.1 2800 25 1.2 0.15 12,500
8.9 50 1.1 0.16 11,000 8.8 75 1.2 0.16 10,500 8.7 100 1.1 0.15
10,500 8.7
EXAMPLE 3
Pyromellitic dianhydride (1 equivalent) and
p-phenylenebis(trimellitic acid monoester anhydride) were dissolved
in a solution prepared by dissolving 1 equivalent of
4,4'-oxydianiline and 1 equivalent of p-phenylenediamine in DMF to
produce a solution containing 15% by weight of polyamic acid.
While cooling the resulting solution, an imidization catalyst
containing 1 equivalent of acetic anhydride and 1 equivalent of
isoquinoline, relative to the carboxylic acid group contained in
the polyamic acid, and DMF was added thereto, followed by
defoaming. Subsequently, the resulting mixed solution was applied
onto an aluminum foil such that a predetermined thickness was
achieved after drying. The mixed solution layer on the aluminum
foil was dried using a hot-air oven and a far-infrared heater.
The drying conditions for achieving a final thickness of 75 .mu.m
were as follows. The mixed solution layer on the aluminum foil was
dried in a hot-air oven at 120.degree. C. for 240 seconds to
produce a self-supporting gel film. The resulting gel film was
stripped off from the aluminum foil and fixed on a frame. The gel
film was dried by heating stepwise in a hot-air oven at 120.degree.
C. for 30 seconds, at 275.degree. C. for 40 seconds, at 400.degree.
C. for 43 seconds, and at 450.degree. C. for 50 seconds, and with a
far-infrared heater at 460.degree. C. for 23 seconds. With respect
to other thicknesses, the firing time was adjusted in proportion to
the thickness. For example, in the case of a film with a thickness
of 25 .mu.m, the firing time was decreased to one third, compared
with the case of 75 .mu.m.
When compared with Example 1, the resulting polyimide films had
higher molecular orientation in Example 3. Consequently, in the
case of thick films, the solvent and the catalyst were caught in
the resin, and foaming easily occurred because of evaporation of
the solvent of the polyimide film and the imidization catalyst. In
order to prevent foaming, it was necessary to set the firing time
at low temperatures to be sufficiently long.
Four types of polyimide films with thicknesses of 25 .mu.m, 50
.mu.m, 75 .mu.m, and 100 .mu.m (Sample C: elastic modulus 500
kg/mm.sup.2, coefficient of water absorption>1.5%) were produce.
Using these films, graphitization was performed also in Example 3
by the same method as that in Example 1.
The properties of the filmy graphites produced in Example 3 are
shown in Table 4. As is evident from comparison between Tables 4
and 1, the properties of the filmy graphites obtained in Example 3
were substantially the same as those in Example 1.
TABLE-US-00004 TABLE 4 Starting film Heat treatment Coefficient of
temperature Thickness linear expansion Electrical conductivity
Thermal diffusivity (.degree. C.) (.mu.m)
(.times.10.sup.-5/.degree. C.) Birefringence (S cm) (10.sup.-4
m.sup.2/s) 2700 25 0.9 0.16 10,000 8.3 50 1.0 0.16 10,000 8.3 75
0.9 0.15 9,500 8.1 100 1.0 0.15 9,300 8.1 2800 25 0.9 0.16 11,300
8.5 50 1.0 0.16 11,000 8.3 75 0.9 0.15 10,000 8.2 100 1.0 0.15
9,500 8.2
EXAMPLE 4
Pyromellitic dianhydride (4 equivalents) was dissolved in a
solution prepared by dissolving 3 equivalents of 4,4'-oxydianiline
and 1 equivalent of p-phenylenediamine in dimethylformamide (DMF)
to produce a solution containing 18.5% by weight of polyamic
acid.
While cooling the resulting solution, an imidization catalyst
containing 1 equivalent of isoquinoline relative to the carboxylic
acid group contained in the polyamic acid and DMF was added
thereto, followed by defoaming. Subsequently, the resulting mixed
solution was applied onto an aluminum foil such that a
predetermined thickness was achieved after drying. The mixed
solution layer on the aluminum foil was dried using a hot-air
oven.
The drying conditions for achieving a final thickness of 75 .mu.m
were as follows. The mixed solution layer on the aluminum foil was
dried in a hot-air oven at 120.degree. C. for 240 seconds to
produce a self-supporting gel film. The resulting gel film was
stripped off from the aluminum foil and fixed on a frame. The gel
film was dried by heating stepwise in a hot-air oven at 120.degree.
C. for 30 minutes, at 275.degree. C. for 30 minutes, at 400.degree.
C. for 30 minutes, and at 450.degree. C. for 30 minutes. With
respect to other thicknesses, the firing time was adjusted in
proportion to the thickness. For example, in the case of a film
with a thickness of 25 .mu.m, the firing time was decreased to one
third, compared with the case of 75 .mu.m. Furthermore, when the
thickness of the film was large, in order to prevent foaming
because of the evaporation of the solvent of the polyimide film and
the imidization catalyst, the firing time at low temperatures was
set to be sufficiently long. In particular, when acetic anhydride
is not added as in Example 4, the reaction proceeds slowly and the
polarity does not change. As a result, the exudation of the solvent
and the catalyst slows down, and foaming easily occurs during the
formation of the polyimide film. Consequently, when a thick
polyimide film is formed, adequate attention must be paid to the
drying conditions.
Four types of polyimide films with thicknesses of 25 .mu.m, 50
.mu.m, 75 .mu.m, and 100 .mu.m (Sample D: elastic modulus 380
kg/mm.sup.2, coefficient of water absorption>2.2%) were
produced. Using these films, graphitization was performed also in
Example 4 by the same method as that in Example 1.
The properties of the filmy graphites produced in Example 4 are
shown in Table 5. As is evident from comparison between Tables 5
and 1, the properties of the filmy graphites obtained in Example 4
are slightly inferior to those in Example 1, but are superior to
those in Comparative Example.
TABLE-US-00005 TABLE 5 Starting film Heat treatment Coefficient of
temperature Thickness linear expansion Electrical conductivity
Thermal diffusivity (.degree. C.) (.mu.m)
(.times.10.sup.-5/.degree. C.) Birefringence (S cm) (10.sup.-4
m.sup.2/s) 2700 25 2.0 0.13 10,500 8.0 50 2.0 0.13 10,000 7.8 75
2.0 0.13 9,200 7.8 100 2.1 0.13 8,000 7.8 2800 25 2.0 0.13 11,500
8.5 50 2.0 0.13 10,500 8.1 75 2.0 0.13 9,500 8.1 100 2.1 0.13 8,500
8.1
EXAMPLE 5
In Example 5, polyimide films manufactured by Kaneka Corporation
and sold under the trade name of APICAL NPI with various
thicknesses were graphitized by the same method as that in Example
1.
APICAL NPI was produced as follows. Pyromellitic dianhydride (4
equivalents) was dissolved in a solution prepared by dissolving 3
equivalents of 4,4'-oxydianiline in DMF to synthesize a pre-polymer
having acid anhydrides at both termini. Subsequently, by dissolving
1 equivalent of p-phenylenediamine in a solution containing the
pre-polymer, a solution containing 18.5% by weight of polyamic acid
was prepared.
While cooling the resulting solution, an imidization catalyst
containing 1 equivalent of isoquinoline relative to the carboxylic
acid group contained in the polyamic acid and DMF was added
thereto, followed by defoaming. Subsequently, the resulting mixed
solution was applied onto an aluminum foil such that a
predetermined thickness was achieved after drying. The mixed
solution layer on a metal belt was dried using a hot-air oven and a
far-infrared heater.
The drying conditions for achieving a final thickness of 75 .mu.m
were as follows. The mixed solution layer on the metal belt was
dried in a hot-air oven at 120.degree. C. for 240 seconds to
produce a self-supporting gel film. The resulting gel film was
stripped off from the metal belt, and the ends of the gel film were
fixed. The gel film was dried by heating stepwise in a hot-air oven
at 120.degree. C. for 30 seconds, at 275.degree. C. for 40 seconds,
at 400.degree. C. for 43 seconds, and at 450.degree. C. for 50
seconds, and with a far-infrared heater at 460.degree. C. for 23
seconds. With respect to other thicknesses, the firing time was
adjusted in proportion to the thickness. For example, in the case
of a film with a thickness of 25 .mu.m, the firing time was
decreased to one third, compared with the case of 75 .mu.m.
Five types of sequential-controlled polyimide films with
thicknesses of 12.5 .mu.m, 25 .mu.m, 50 .mu.m, 75 .mu.m, and 125
.mu.m (Sample E: elastic modulus 380 kgf/mm.sup.2, coefficient of
water absorption 2.2%, birefringence 0.14, coefficient of linear
expansion 1.6.times.10.sup.-5/.degree. C.) were produced. Using
these films, graphitization was performed also in Example 5 by the
same method as that in Example 1 except that the heat treatment
temperature was set at 2,800.degree. C. or 3,000.degree. C. in
Example 5.
Various physical properties of the filmy graphites obtained in
Example 5 are shown in Table 6. As is evident from comparison
between Table 6 and Tables 1 to 5, the properties of the filmy
graphites in Example 5 are superior not only to those of
Comparative Example 1 but also to those of Examples 1 to 4. Of
course, as graphitization proceeds, the density and electrical
conductivity tend to increase. In Table 6, .rho.(77K)/.rho.(rt) and
.rho.(4K)/.rho.(rt) respectively indicate the ratio of electrical
resistance at 77 K to that at room temperature and the ratio of the
electrical resistance at 4 K to that at room temperature. These
electrical resistance ratios tend to decrease as graphitization
proceeds.
Furthermore, the thermal conductivity and the thermal diffusivity
tend to increase as graphitization proceeds, and these properties
are directly important when the filmy graphite is used as a
heat-dissipating film. Here, the thermal conductivity (W/(mK)) was
calculated by multiplying the thermal diffusivity (m.sup.2/s) times
the density (kg/m.sup.3), and times the specific heat (theoretical
value: 0.709 kJ/(kgK)). The density was calculated by dividing the
weight by the volume.
The Raman spectrum intensity ratio in Table 6 is represented by the
ratio of the spectrum peak at a wave number of 1,310 cm.sup.-1
corresponding to the diamond bond to the spectrum peak at a wave
number of 1,580 cm.sup.-1 corresponding to the graphite bond. Of
course, the lower spectrum peak ratio indicates higher
graphitization. In the Raman measurement, a light beam with a
diameter of 10 .mu.m was applied to the center of a cross section
in the thickness direction of the filmy graphite.
TABLE-US-00006 TABLE 6 Properties of filmy graphite Heat Thickness
Raman treatment of starting Electrical Thermal Thermal spectrum
intensity temperature material Thickness Density conductivity .rho.
(77 K) .rho. (4 K) conductivity diffusivity ratio (.degree. C.)
(.mu.m) (.mu.m) (g/cm.sup.3) (S cm) .rho. (rt) .rho. (rt) (W/m K)
(.times.10.sup.-4 m.sup.2/s) (1310 cm.sup.-1/1580 cm.sup.-1) 2800
12.5 5.0 2.2 13500 -- -- 1481 9.5 -- 25 10.3 2.19 13000 0.9 0.75
1398 9 -- 50 21 2.17 13000 1.0 0.9 1305 9 -- 75 33 2.14 12000 1.3
1.2 1500 9.9 0.38 125 55 2.10 12000 1.45 1.4 1486 10 0.41 3000 12.5
4.9 2.42 20000 -- -- 1767 10.3 -- 25 10.6 2.41 20000 0.8 0.4 1760
10.3 -- 50 20 2.38 16000 0.9 0.75 1725 10.2 -- 75 32.5 2.17 16000
1.1 1 1536 10 0.26 125 55.5 2.12 13000 1.2 1 1505 10 0.3
FIG. 3 is a bright-field image of a filmy graphite in the vicinity
of a surface layer, the filmy graphite being obtained by heat
treatment at 3,000.degree. C. of the polyimide film with a
thickness of 125 .mu.m in Example 5, the image being observed with
a transmission electron microscope (TEM). In the TEM observation,
the filmy graphite was embedded in a protective resin to prepare a
specimen for observing a cross section in the thickness direction
of the graphite layer. In FIG. 3, arrows indicate a boundary
between the protective resin and the graphite layer.
As is evident from the layered contrast of the TEM photograph, the
graphite layer has a single-crystal structure in which the
crystallographic (0001) plane (also referred to as the "c-plane",
in general) is parallel to the surface. In FIG. 3, delaminations
along the c-plane are observed. These delaminations are caused by
unexpected external force during the preparation and handling of
the specimen for the microscope. The fact that such delaminations
easily occur means that graphite crystallization has proceeded to a
high degree and breaking easily occurs along the c-plane.
FIG. 4 is a crystal lattice image observed with a TEM in the
vicinity of the surface layer corresponding to FIG. 3. In FIG. 4, a
linear lattice image corresponding to the c-plane of the graphite
is shown with a clear contrast and it can be confirmed that the
linear lattice extends parallel to the surface.
FIG. 5, similar to FIG. 3, is a bright-field image of a filmy
graphite in the vicinity of a center in the thickness direction,
the image being observed with a TEM. As is evident from the TEM
photograph, the same graphite single-crystal structure as that of
the surface layer is also formed in the vicinity of the center in
the thickness direction. In FIG. 5, similar to FIG. 3,
delaminations along the c-plane are observed. FIG. 6 is a crystal
lattice image observed with a TEM in the vicinity of the center in
the thickness direction corresponding to FIG. 5. In FIG. 6,
although linearity of the linear lattice image corresponding to the
c-plane of the graphite is slightly inferior to that in FIG. 4,
extending of the lines can be confirmed.
COMPARATIVE EXAMPLE 2
In Comparative Example 2, polyimide films manufactured by DuPont
and sold under the trade name of Kapton H with various thicknesses
were graphitized by the same method as that in Example 5.
Although the production process of the Kapton film is not known, it
is assumed that the Kapton film is produced by a method in which an
imidization catalyst composed of acetic anhydride, beta-picoline,
and DMAc is added to a polyamic acid solution prepared by
dissolving 1 equivalent of 4,4'-oxydianiline in dimethylacetamide
(DMAC) and further dissolving 1 equivalent of pyromellitic
dianhydride therein.
The Kapton H has an elastic modulus of 330 kgf/mm.sup.2, a
coefficient of water absorption of 2.9%, a birefringence of 0.11,
and a coefficient of linear expansion of
2.7.times.10.sup.-5/.degree. C.
Various physical properties of the filmy graphites obtained in
Comparative Example 2 are shown in Table 7. As is evident from
comparison between Tables 7 and 6, the properties of the filmy
graphites in Example 5 are remarkably superior to the filmy
graphites produced from the Kapton films in Comparative Example
2.
TABLE-US-00007 TABLE 7 Properties of filmy graphite Heat Thickness
Raman treatment of starting Electrical Thermal Thermal spectrum
intensity temperature material Thickness Density conductivity .rho.
(77 K) .rho. (4 K) conductivity diffusivity ratio (.degree. C.)
(.mu.m) (.mu.m) (g/cm.sup.3) (S cm) .rho. (rt) .rho. (rt) (W/m K)
(.times.10.sup.-4 m.sup.2/s) (1310 cm.sup.-1/1580 cm.sup.-1) 2800
25 10.3 2.27 11000 1.1 1 887 5.7 -- 50 23.5 2.13 6900 1.2 1.1 680
4.5 -- 75 41 1.89 3500 1.8 1.6 442 3.3 0.41 125 100 1.42 980 2.5
2.8 238 2.3 0.44 3000 25 10.1 2.46 18000 1.05 0.8 1706 9.8 -- 50 21
2.38 12000 1.2 1.1 1519 9.0 -- 75 35 2.14 8000 1.7 1.5 1237 8.1
0.37 125 70 1.86 2800 2.1 2.5 660 5 0.4
FIG. 7 is a bright-field image of a filmy graphite in the vicinity
of a surface layer, the filmy graphite being obtained by heat
treatment at 3,000.degree. C. of the Kapton (registered trademark)
film with a thickness of 125 .mu.m in Comparative Example 2, the
image being observed with a TEM. In FIG. 7, a single-crystal
structure in which the c-plane of the graphite is parallel to the
surface occurs in the vicinity of the surface layer. FIG. 8 is a
crystal lattice image observed with a TEM in the vicinity of the
surface layer corresponding to FIG. 7. In FIG. 8, although
linearity of the linear lattice image corresponding to the c-plane
of the graphite is slightly inferior to that in FIG. 4, extending
of the lines can be confirmed.
FIG. 9 is a bright-field image of a filmy graphite in the vicinity
of a center in the thickness direction, the image being observed
with a TEM. In the TEM photograph, a layered structure is not
formed in the vicinity of the center in the thickness direction
unlike the surface layer, and thus it is evident that
graphitization is insufficient. FIG. 10 is a crystal lattice image
observed with a TEM in the vicinity of a center in the thickness
direction corresponding to FIG. 9. In FIG. 10, it can be confirmed
that the linear lattice image corresponding to the c-plane of the
graphite is wavy and broken. This indicates that even if
graphitization partially proceeds, the resulting graphite is in a
microcrystalline state or the c-plane orientation is not aligned.
That is, with respect to the filmy graphite produced from the
Kapton (registered trademark) film, even if heat treatment is
performed at a high temperature of 3,000.degree. C., graphitization
does not sufficiently proceed in the center in the thickness
direction.
INDUSTRIAL APPLICABILITY
According to the present invention, in comparison with the
conventional polymer graphitization process, it is possible to
produce a thicker filmy graphite, and graphitization is enabled at
lower temperatures and in a shorter time when a polymer film with
the same thickness is graphitized.
* * * * *